Remove @backtrac_string from context array

Problem/Motivation

Follow up from #3175449: Display backtrace for logged throwables on log message details page

@xjm @alexpott Looking at core/lib/Drupal/Core/EventSubscriber/ExceptionLoggingSubscriber.php I do see one place where we could consider removing @backtrace_string from the context array: The ExceptionLoggingSubscriber::onClientError() method.

It's already removed in the ExceptionLoggingSubscriber::on403() method.

This would clean up the UI by removing the backtrace for miscellaneous 4xx errors (e.g. \Symfony\Component\HttpKernel\Exception\UnsupportedMediaTypeHttpException) logged by this mechanism, and would reduce storage size of dblog module going forward, for sites that have been getting lots of non-404/403 4xx errors.

Steps to reproduce

Proposed resolution

TBD

Remaining tasks

Determine if we should remove - IMPORTANT
Review
Review

User interface changes

NA

API changes

NA

Data model changes

NA

Release notes snippet

NA